In multi-media work station displays, it is known that two independent rasters, such as a standard TV video and high resolution computer generated graphics video may each be displayed on a high resolution graphics monitor by the use of dual frame buffers. A TV frame buffer includes a dual port VRAM, with the serial and random ports operating asynchronously. The primary port receives incoming TV video synchronously as it comes in, and the secondary port reads the TV video out synchronously with the high resolution graphics monitor. A high resolution frame buffer in a computer is utilized to store high resolution graphics which is read out synchronously with the high resolution graphics monitor. A switching mechanism selects which of the TV video and the high resolution graphics video is to be displayed at a given time. The TV frame buffer includes an on screen and off screen portion. The computer provides computer data, including high resolution graphics data and audio data to the TV frame buffer, with the graphics data being stored in the on screen portion and the audio data being stored in the off screen portion. The audio data is read out to an audio circuit for replay. The graphics data is combined with the TV video for purposes of windowing.
Dual buffers of this type are costly on both space and production. Also, it is difficult to edit the merged graphics and video for separate frames.
An example of a dual frame buffer display system is described in U.S. Pat. No. 4,994,912 issued Feb. 19, 1991 to Lumelsky et al. and entitled AUDIO VIDEO INTERACTIVE DISPLAY.
In addition to dual frame buffers having cost and space and graphics/video data merger drawbacks, it is difficult to extend displays in dual frame buffer systems for higher spatial resolution or higher frame refresh rate and it is difficult to develop drivers for inconsistent buffer sizes, different data formats and the like.
There are several other previously known approaches to combining video with high resolution graphics. Some methods double the scan rate of the incoming video through the use of a line buffer, reading out each video line twice for each line of the high-resolution screen. This method has several drawbacks. First, it assumes that the high-resolution display is exactly twice the scan rate of th incoming video. This is seldom the case, and always requires a gen-lock circuit at the very least to force this strict relationship between the video and the graphics. It also fails to provide random access to the video information from the host workstation, since there is no frame buffer to store the video information. Another method involves converting the video and graphics information into a common format and storing the two into a single, common frame buffer. While this may at first seem to be an advantage in that only one frame buffer is needed, this buffer requires far more memory than having two separate dedicated buffers. In the prior art, in order to use a single frame buffer to store both types of visual data, a very large frame buffer is required that is both "wide" and "deep".